简介
Semaphore（信号量）是用来控制同时访问特定资源的线程数量，它通过协调各个线程，以保证合理的使用公共资源。很多年以来，我都觉得从字面上很难理解Semaphore所表达的含义，只能把它比作是控制流量的红绿灯，比如XX马路要限制流量，只允许同时有一百辆车在这条路上行使，其他的都必须在路口等待，所以前一百辆车会看到绿灯，可以开进这条马路，后面的车会看到红灯，不能驶入XX马路，但是如果前一百辆中有五辆车已经离开了XX马路，那么后面就允许有5辆车驶入马路，这个例子里说的车就是线程，驶入马路就表示线程在执行，离开马路就表示线程执行完成，看见红灯就表示线程被阻塞，不能执行。
示例
下面用一个示例演示，假设有N个并发线程都要打印文件，但是打印机只有1台，先来一个打印队列类：
import java.util.concurrent.Semaphore;
public class PrintQueue {
private final Semaphore semaphore;
public PrintQueue() {
semaphore = new Semaphore(1);//限定了共享资源只能有1个（相当于只有一把钥匙）
}
public void printJob(Object document) {
try {
semaphore.acquire();//取得对共享资源的访问权（即拿到了钥匙））
long duration = (long) (1 + Math.random() * 10);
System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n", Thread.currentThread().getName(), duration);
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
semaphore.release();//钥匙用完了，要还回去，这样其它线程才能继续有序的拿到钥匙，访问资源
}
}
}由于是在多线程环境中，真正运行的作业处理，得继承自Runnable(或Callable)
public class Job implements Runnable {
private PrintQueue printQueue;
public Job(PrintQueue printQueue) {
this.printQueue = printQueue;
}
public void run() {
System.out.printf("%s: Going to print a job\n", Thread.currentThread().getName());
printQueue.printJob(new Object());
System.out.printf("%s: The document has been printed\n", Thread.currentThread().getName());
}
}好了，测试一把：
public class Main {
public static void main(String args[]) {
PrintQueue printQueue = new PrintQueue();
int threadCount = 3;
Thread thread[] = new Thread[threadCount];
for (int i = 0; i < threadCount; i++) {
thread[i] = new Thread(new Job(printQueue), "Thread" + i);
}
for (int i = 0; i < threadCount; i++) {
thread[i].start();
}
}
}输出：
Thread0: Going to print a job
Thread2: Going to print a job
Thread1: Going to print a job
Thread0: PrintQueue: Printing a Job during 7 seconds
Thread0: The document has been printed
Thread2: PrintQueue: Printing a Job during 5 seconds
Thread2: The document has been printed
Thread1: PrintQueue: Printing a Job during 1 seconds
Thread1: The document has been printed从输出上看，线程0打印完成后，线程2才开始打印，然后才是线程1，没有出现一哄而上，抢占打印机的情况。这样可能没啥感觉，我们把PrintQueue如果去掉Semaphore的部分，变成下面这样：
public class PrintQueue {
//private final Semaphore semaphore;
public PrintQueue() {
//semaphore = new Semaphore(1);//限定了共享资源只能有1个（相当于只有一把钥匙）
}
public void printJob(Object document) {
try {
//semaphore.acquire();//取得对共享资源的访问权（即拿到了钥匙））
long duration = (long) (1 + Math.random() * 10);
System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n", Thread.currentThread().getName(), duration);
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
//semaphore.release();//钥匙用完了，要还回去，这样其它线程才能继续有序的拿到钥匙，访问资源
}
}
}这回的输出：
Thread0: Going to print a job
Thread2: Going to print a job
Thread1: Going to print a job
Thread2: PrintQueue: Printing a Job during 4 seconds
Thread1: PrintQueue: Printing a Job during 8 seconds
Thread0: PrintQueue: Printing a Job during 1 seconds
Thread0: The document has been printed
Thread2: The document has been printed
Thread1: The document has been printed可以发现，3个线程全都一拥而上，同时开始打印，也不管打印机是否空闲，实际应用中，这样必然出问题。
好的，继续，突然有一天，公司有钱了，又买了2台打印机，这样就有3台打印机了，这时候怎么办呢？简单的把PrintQueue构造器中的
public PrintQueue() {
semaphore = new Semaphore(3);
}就行了吗？仔细想想，就会发现问题，代码中并没有哪里能告诉线程哪个打印机正在打印，哪个打印机当前空闲，所以仍然有可能出现N个线程(N<=3)同时抢一台打印机的情况（即：如果把控制权当成钥匙的话，相当于有可能3个人各领取到了1把钥匙，但是这3把钥匙是相同的，3个人都看中了同一个箱子，都要用手中的钥匙去抢着开箱）。
所以得改进一下：
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PrintQueue {
private boolean freePrinters[];//用来存放打印机的状态，true表示空闲，false表示正在打印
private Lock lockPrinters;//增加了锁，保证多个线程，只能获取得锁，才能查询哪台打印机空闲的
private final Semaphore semaphore;
public PrintQueue() {
int printerNum = 3;//假设有3台打印机
semaphore = new Semaphore(printerNum);
freePrinters = new boolean[printerNum];
for (int i = 0; i < printerNum; i++) {
freePrinters[i] = true;//初始化时，默认所有打印机都空闲
}
lockPrinters = new ReentrantLock();
}
private int getPrinter() {
int ret = -1;
try {
lockPrinters.lock();//先加锁，保证1次只能有1个线程来获取空闲的打印机
for (int i = 0; i < freePrinters.length; i++) {
//遍历所有打印机的状态，发现有第1个空闲的打印机后，领取号码，
// 并设置该打印机为繁忙状态（因为马上就要用它）
if (freePrinters[i]) {
ret = i;
freePrinters[i] = false;
break;
}
}
} catch (Exception e) {
e.printStackTrace();
} finally {
//最后别忘记了解锁，这样后面的线程才能上来领号
lockPrinters.unlock();
}
return ret;
}
public void printJob(Object document) {
try {
semaphore.acquire();
int assignedPrinter = getPrinter();//领号
long duration = (long) (1 + Math.random() * 10);
System.out.printf("%s: PrintQueue: Printing a Job in Printer%d during %d seconds\n", Thread.currentThread().getName(),
assignedPrinter, duration);
Thread.sleep(duration);
freePrinters[assignedPrinter] = true;//打印完以后，将该打印机重新恢复为空闲状态
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
semaphore.release();
}
}
}测试一下，这回把线程数增加到5，输出结果类似下面这样：
Thread0: Going to print a job
Thread4: Going to print a job
Thread3: Going to print a job
Thread2: Going to print a job
Thread1: Going to print a job
Thread4: PrintQueue: Printing a Job in Printer1 during 7 seconds
Thread0: PrintQueue: Printing a Job in Printer0 during 4 seconds
Thread3: PrintQueue: Printing a Job in Printer2 during 8 seconds
Thread0: The document has been printed
Thread2: PrintQueue: Printing a Job in Printer0 during 1 seconds
Thread2: The document has been printed
Thread4: The document has been printed
Thread1: PrintQueue: Printing a Job in Printer0 during 1 seconds
Thread3: The document has been printed
Thread1: The document has been printed从输出结果可以看出，一次最多只能有3个线程使用这3台打印机，而且每个线程使用的打印机互不冲突，打印完成后，空闲的打印机会给其它线程继续使用，继续折腾，如果把getPrinter()中加锁的部分去掉，即：
private int getPrinter() {
int ret = -1;
try {
//lockPrinters.lock();//先加锁，保证1次只能有1个线程来获取空闲的打印机
for (int i = 0; i < freePrinters.length; i++) {
//遍历所有打印机的状态，发现有第1个空闲的打印机后，领取号码，
// 并设置该打印机为繁忙状态（因为马上就要用它）
if (freePrinters[i]) {
ret = i;
freePrinters[i] = false;
break;
}
}
} catch (Exception e) {
e.printStackTrace();
} finally {
//最后别忘记了解锁，这样后面的线程才能上来领号
//lockPrinters.unlock();
}
return ret;
}再跑一下，结果如何，为了放大冲突，这回开到15个线程来抢3台打印机，输出如下：
Thread0: Going to print a job
Thread14: Going to print a job
Thread13: Going to print a job
Thread12: Going to print a job
Thread11: Going to print a job
Thread10: Going to print a job
Thread9: Going to print a job
Thread8: Going to print a job
Thread7: Going to print a job
Thread6: Going to print a job
Thread5: Going to print a job
Thread4: Going to print a job
Thread3: Going to print a job
Thread2: Going to print a job
Thread1: Going to print a job
Thread0: PrintQueue: Printing a Job in Printer0 during 29 seconds
Thread14: PrintQueue: Printing a Job in Printer0 during 92 seconds
Thread13: PrintQueue: Printing a Job in Printer1 during 66 seconds
Thread0: The document has been printed
Thread12: PrintQueue: Printing a Job in Printer0 during 86 seconds
Thread13: The document has been printed
Thread11: PrintQueue: Printing a Job in Printer1 during 1 seconds
Thread11: The document has been printed
Thread10: PrintQueue: Printing a Job in Printer1 during 58 seconds
Thread14: The document has been printed
Thread9: PrintQueue: Printing a Job in Printer0 during 92 seconds
Thread12: The document has been printed
Thread8: PrintQueue: Printing a Job in Printer0 during 59 seconds
Thread10: The document has been printed
Thread7: PrintQueue: Printing a Job in Printer1 during 51 seconds
Thread8: The document has been printed
Thread6: PrintQueue: Printing a Job in Printer0 during 33 seconds
Thread7: The document has been printed
Thread5: PrintQueue: Printing a Job in Printer1 during 2 seconds
Thread9: The document has been printed
Thread3: PrintQueue: Printing a Job in Printer1 during 85 seconds
Thread4: PrintQueue: Printing a Job in Printer0 during 61 seconds
Thread5: The document has been printed
Thread6: The document has been printed
Thread2: PrintQueue: Printing a Job in Printer0 during 66 seconds
Thread4: The document has been printed
Thread1: PrintQueue: Printing a Job in Printer0 during 9 seconds
Thread1: The document has been printed
Thread3: The document has been printed
Thread2: The document has been printed
注意标粗的部分：Thread0与Thread14同时分配到了Printer0上了，出现了多个线程同时抢一个资源的情况。